Optical fibers are key components in modern telecommunications systems. Basically, an optical fiber is a thin strand of glass capable of transmitting optical signals containing a large amount of information over long distance with low loss. Advantageously the fiber transmits all wavelengths of interest with the same low loss.
In its simplest form, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction peripherally surrounded by a cladding having a second (lower) index of refraction. Typically optical fibers are composed of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical fibers are typically fabricated by a two-step process that involves, as a first step, the fabrication of a specially constructed glass rod called a preform, and, as a second step, drawing the preform into the fiber.
Preforms are commercially fabricated using thermal chemical vapor reactions that form mixed oxides. The oxides are deposited as layers of glass soot (particles) onto a rotating mandrel or a high purity tube. The deposited soot is then consolidated by a sintering process and collapsed into a clear preform for fiber draw. The most commonly used commercial preform fabrication processes are 1) modified chemical vapor deposition (MCVD), outside vapor deposition (OVD) and vapor phase axial deposition (VAD).
The MCVD process differs from OVD and VAD in that the vapor deposition in MCVD occurs on the inside surface of the glass starting tube. The preform is built up from the outside to the inside by forming glassy layers of doped silica particles on the inside surface of the starting tube. Upon completing the various deposited layers, the tube is collapsed into a solid rod. Further details concerning the MCVD process can be found in U.S. Pat. Nos. 4,909,816; 4,217,027 and 4,334,903 issued to MacChesney et al. and incorporated herein by reference.
In the OVD process the soot stream deposits on the outer surface of a mandrel and builds up radially to form a porous body. After the mandrel is removed, the body is inserted into a consolidation furnace where it is dried and sintered. Further details concerning the OVD process are set forth in U.S. Pat. Nos. 3,375,075 and 3,826,560 which are incorporated herein by reference.
In the VAD process the soot stream deposits on the end of a target rod and builds up axially to form a porous body similar to that produced by the OVD process except that the VAD body has no axial aperture. The VAD body is dried, sintered to dense glass and drawn to fiber. Further details concerning the VAD process are set forth in T. Izawa et al “Continuous fabrication of high silica fiber preform”, IOOC '71, C1-1, pp. 375-378, July 1977.
These three major processes (MCVD, OVD and VAD) all use similar chemical delivery systems to deposit glass soot. The most abundant reagent is silicon tetrachloride (SiCl4) which reacts with oxygen to form silica (SiO2). Small amounts of other compounds are used to dope the silica and change its refractive index. Germanium tetrachloride (GeCl4) and phosphorous oxychloride (POCl3) are used to raise the refractive index above silica. Freons (e.g. CF3Cl, CF4, CF3Br, C2F6 or CCl2F2), are often used to lower the index of refraction. During the collapse phase of preform production, carbon tetrachloride (CCl4) or chlorine (Cl2) is often introduced as a drying agent in order to maintain low water levels.
As mentioned above, it is highly desirable that the optical fiber produced by these processes should transmit wavelengths of interest with substantially uniform low loss. Unfortunately, as the range of utilized wavelengths have extended into the L-band, uniform low loss is not the case.